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Rainfall and Runoff

For the month of November, rainfall amounts for the part of the Valley above Chattanooga were 176% of normal. For the rest of the Valley, November rainfall totaled 144% of normal.

Rainfall amounts kept increasing steadily during the month, with each week wetter than the previous one. The last week in November brought 2.5 inches of rain to the Tennessee River basin, which was 163% of normal for the month.

This is the time of year when we start to see an increase in the amount of runoff, in relation to rainfall. Evapotranspiration rates are lower, due to lower temperatures and decreased plant activity. When vegetation goes dormant, the amount of moisture it takes up decreases. So more rainfall runs off (eventually, into the reservoirs) instead of being absorbed.

For the part of the Valley above Chattanooga, runoff totals for November were 241% of normal, with the rest of the Valley coming in at 188% of normal for the month. For the entire basin, runoff totaled 221% of normal for November.

Current Reservoir Conditions

Watauga, Douglas, Fontana, Cherokee, and Chatuge Reservoirs are all at or near flood guide. Norris Reservoir was above flood guide at the end of November, due to heavy rainfall in that watershed during the last two weeks of the month.

The main-river reservoirs are all above their normal winter operating ranges—with the exception of Nickajack and Wilson, which are within their winter operating zones.

Tributary Reservoir Elevations

Reservoir

December 1, 2011
Observed Elevation1

December 1 Flood Guide2

January 1 Flood Guide2

South Holston

1707.5

1712.1

1708.0

Watauga

1952.9

1953.4

1952.0

Cherokee

1047.2

1049.7

1045.0

Douglas

962.5

961.4

954.0

Fontana

1660.7

1660.1

1653.0

Norris

1007.8

1003.9

1000.0

Chatuge

1918.8

1918.8

1918.0

Nottely

1762.0

1763.4

1762.0

Hiwassee

1486.9

1493.3

1485.0

Blue Ridge

1670.0

1671.6

1668.0

Tims Ford

883.0

879.0

873.0

Normandy

868.1

865.0

864.0

¹Elevations above mean sea level, as of 12:01 a.m. on this date

²Flood guide levels show the amount of storage allocated for flood damage reduction during different times of the year. During the summer, TVA's goal is to meet downstream flow requirements while keeping the reservoir level at the dam as close to the flood guide level as possible to support reservoir recreation. From June 1 through Labor Day, reservoir levels fall below the flood guide only when rain and runoff are insufficient to meet flow requirements. During the rest of the year, the primary objective is to keep the reservoir level at or below the flood guide to ensure there is enough space in the reservoir to store the rain and runoff from flood events.

Reservoir Operations

Sometimes, it can seem like “one step forward, two steps back.” According to TVA’s Senior Manager of Forecast Operations Tom Barnett, it’s a continuous effort to stay abreast of rainfall events—particularly this time of year.

“Although floods can occur at any time of year, the month of December typically marks the beginning of flood season for the Tennessee Valley,” he explains. “Consequently, our focus right now is on recovering and maintaining storage space in the tributary reservoirs. That can be hard to do at times, especially when we get back-to-back rains like we’ve been experiencing here lately.”

Spill operations at Wheeler, as well as at almost all of the other main-river reservoirs, have been necessary in order to help recover much-needed storage space at the beginning of flood season for the Tennessee Valley.

Barnett and the others who staff TVA’s River Forecast Center have been working intently to find just the right balance between holding water in the tributary reservoirs (to minimize downstream flooding) and releasing it, in order to make space available to store additional rainfall and runoff that is certain to come.

“Toward the end of November,” explains Barnett, “we ended up storing more water in the tributary reservoirs, in an effort to reduce downstream flooding. This was a prime example of using the tributary storage system for its intended purpose.”

Because of recent heavy rainfall in the Norris watershed, elevations at that project are presently above flood guide. “We’ll be attempting to recover that flood storage as soon as practical,” he says. “If we get a break from these rainfall events that have been coming one right after the other, our job will get a lot easier.”

Currently, spill operations are occurring at all main-river projects, with the exception of Fort Loudoun. But make no mistake: storage space recovery never takes place without running water through the turbines at maximum capacity. According to Barnett, “That’s the first rule and an over-arching theme of our operations. We challenge ourselves to get the absolute most from every drop of water we move through the system. Our responsibility is to make the best possible use of the resource, and that means using releases to produce hydropower, whenever we can, while at the same time providing other benefits.”

Calculating Flood Damages Averted

A way to quantify just how much worse it might have been

Throughout history, many areas throughout the Tennessee Valley experienced significant damage from frequent flooding. These same locations are still prone to flooding, but—thanks to flood reduction benefits provided by the TVA reservoir system—widespread devastation from these events is largely a thing of the past.

It’s not easy to put a dollar figure on what didn’t happen. But that’s precisely what Karen Ford is called upon to do, in the wake of each flood-producing storm that affects communities along the Tennessee River and its tributaries. In her work with TVA’s Flood Risk Reduction program, Ford is responsible for looking very closely at a range of factors that are considered when calculating flood damages averted.

“We are tasked with building a hypothetical scenario that re-creates what would most likely have taken place, if it weren’t for the system of TVA dams that serve to minimize impacts from flooding,” she explains. “First, we look at the actual flood crest—the high-water mark, if you will. Then we run a computer program—inputting rainfall totals and data from stream gauges—which tells us how high the crest would have been, without regulation from the reservoir system. What we call our ‘potential flood damage analysis’ reflects the results of field surveys that take into account the features and structures of the properties that would have been affected—whether residential, commercial, industrial, or agricultural.”

Ford and her co-workers enter this information into a computer program that generates a “potential damage curve,” a visual representation of what can be thought of as the flood depth/damage relationship. Basically, it plots the damages that would have occurred (expressed in terms of values, based on current dollars) relative to the estimated depth the floodwaters would have reached. It provides an enlightening—and often sobering—look at what “might have been.”

In many cases, the resulting figure is truly staggering. As tragic as it may be for folks whose property has been flooded, what’s abundantly clear is that the destruction would have been much greater, had it not been for the measure of protection afforded by the reservoir system. In the Chattanooga area alone, the TVA reservoir system has—since the closure of Norris Dam in 1936—averted an estimated $4.9 billion in flood-related damages. If you widen the scope of flood crest reductions to include the rest of the Tennessee Valley, the figure grows to $5.4 billion; add to that the damages that didn’t occur to locations along the Lower Ohio and Mississippi Rivers because of TVA’s capacity for storing floodwaters and you reach a total of $5.8 billion in flood damages averted.

The most destructive flood to impact the Tennessee Valley during the past century occurred in March of 1973. Flood stage in Chattanooga reached 36.9 feet—roughly equivalent to a 100-year flood. However, the natural (or unregulated) flood stage would have been 15.5 feet higher. It was a widespread event, affecting the entire Tennessee Valley watershed. By the time it was over, it had resulted in property damages of approximately $35 million at Chattanooga—but TVA’s ability to reduce the flood crest averted about $465 million in flood damages. In today’s dollars, that would translate to an astounding $1.6 billion in property losses avoided—just from the potential impacts of that single flood event.

This map shows the locations throughout the Valley that directly benefit from TVA’s operation of the reservoir system to reduce flooding. Historically, these were places that experienced flooding on a regular basis. With the capacity for storing floodwaters provided by TVA’s tributary reservoirs, the frequency and magnitude of flooding in these locations has been greatly reduced.

It may seem very much like an academic exercise: running all the computer software, crunching the numbers, and attempting to show, in dollars, the losses associated with flooding that—in point of fact—never actually occurred. But calculating flood damages averted serves an important purpose, namely that of underscoring the value of the reservoir system. “As time passes,” says Ford, “we tend to forget what things were like before TVA built these dams. People farmed the rich land along the river bottoms and built their homes nearby. There was no protection for these folks when floods occurred; their land and their communities were inundated. It’s hard for us to imagine just how vulnerable they were back then.”

It’s certainly true that no system of water control structures can prevent flooding completely, and flood-prone locations along the Tennessee River are still impacted from time to time. But there’s no question that the TVA reservoir system has afforded a vitally important measure of protection from the scope and scale of destruction that used to occur on a regular basis.

“When we evaluate metrics, conduct our potential flood damage analysis, and come up with the dollar figures associated with our projection of flood damages averted,” says Ford, “it’s much more than just generating data based on a hypothetical situation. We may be working in the abstract, admittedly, but in the back of our minds we are always conscious of the fact that these numbers represent real losses that absolutely would have occurred, had it not been for the flood reduction capability of our dams. Put another way, we think about what else would have been under water. That’s huge. TVA is, without question, enhancing the quality of life for Valley citizens by managing the reservoir system to help minimize impacts from flooding. It’s something we take enormous pride in, every day.”

Fast Facts:

TVA reservoirs provide more than 10 million acre-feet of flood control storage during flood “season,” which runs from the beginning of December through the middle of March. This is equal to one foot of water covering 10 million acres of land.

When designing the reservoir system to provide adequate storage for floodwaters, TVA looked at a storm that occurred back in January of 1937 on the Ohio River Valley—using it as a benchmark for the kind of event that the new system would be built to withstand.

Of all locations throughout the Valley, Chattanooga is most at risk from flooding—primarily because it’s situated at the opening of a narrow gorge where the Tennessee River flows through the Cumberland Mountains. The drainage area (or “watershed”) above Chattanooga is vast; even with a TVA dam located on each of the five major tributary rivers that contribute flows into the Tennessee River above the city, the area is still vulnerable to flooding.

There’s a wealth of information contained within the field surveys TVA conducts to document properties at risk of flooding. For example, a residence is listed by structure type, construction, address, river mile location, floor elevation, and elevation of the potential first entry point for rising water. Tax or real estate records are used to determine the property’s value, tables are used to calculate the value of contents, and 20% is added to cover intangibles such as loss of work time, transportation difficulties, loss of power, and other impacts likely to occur during a flood event.

Get more information on TVA.com

The links below will take you to reservoir-related information on TVA’s website.

TVA’s reservoir operating policy: Learn how TVA manages the flow of water through the Tennessee River system to provide navigation, flood damage reduction, power supply, water quality, water supply, recreation, and other benefits.

Reservoir information: Get detailed information about individual reservoirs, including observed and predicted elevations and releases at TVA dams, reservoir operating guides, water quality improvements, fish population survey results, and more.

Rainfall and stream flows: Get the latest information on daily rainfall and stream flows across the Valley.

Map of TVA reservoirs and power plants: Our interactive map is your guide to the entire TVA power system, including fossil and nuclear plants, dams and reservoirs, and visitor centers. You’ll find interesting facts about each facility and learn how they work together for the purposes of power supply, river management, and economic development.

Water supply FAQs: Get answers to frequently asked questions about obtaining a water intake permit, improving water quality around intakes, inter-basin transfers, and more.

Dangerous areas around TVA dams: If you like fishing or enjoy swimming and boating on TVA-managed reservoirs, you need to be aware of the possible hazards around dams, locks, and powerhouses.

How to lock through: Find out what you need to do to safely approach a navigation lock, secure your boat in the lock chamber, and exit the lock.